IMO the hardest part is the legal side, and in fact I’m not very clear how MS skirted that issue other than through US lax enforcement on corporations. In order to have a db like this one must store stuff that is, ordinarily, illegal to store. Because of the use of imperfect, so-called perceptual hashes, and in case of algorithm updates, I don’t think one can get away with simply storing the hash of the file. Some kind of computer vision/AI-ish solution might work out, but I wouldn’t want to be the person compiling that training set…